You should not have to actually wait for the tank to click the portal and enter the twilight realm. It's their job to get in there as fast as possible. Even if you beat the tank in, the dragon does not attack for a few seconds. This isn't like Sunwell where only one could go through the portal at a time and order is important. While the tank and healers getting in fast is important, it's the jobs of each individual to get in fast and start performing their role. The only people who have to actually wait are DPS for a moment while aggro is established, but at that time DPS can already be in the twilight realm getting into position so they are ready to begin DPSing as soon as aggro is established.
There is only one exception to this. If your twilight realm tank gets the debuff right as the phase transition is occuring and needs to move away from the boss to drop the void zone. This is the only reason you should ever delay entering the twilight realm because the tank had to move away from where the portal is. At any other time they should just be entering immediately, along with anyone else going.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.